Re: [PATCH v4 3/4] mm: Optimize mprotect() by PTE-batching
From: Ryan Roberts
Date: Tue Jul 01 2025 - 03:39:46 EST
On 01/07/2025 06:47, Dev Jain wrote:
>
> On 30/06/25 4:01 pm, Ryan Roberts wrote:
>> On 28/06/2025 12:34, Dev Jain wrote:
>>> Use folio_pte_batch to batch process a large folio. Reuse the folio from
>>> prot_numa case if possible.
>>>
>>> For all cases other than the PageAnonExclusive case, if the case holds true
>>> for one pte in the batch, one can confirm that that case will hold true for
>>> other ptes in the batch too; for pte_needs_soft_dirty_wp(), we do not pass
>>> FPB_IGNORE_SOFT_DIRTY. modify_prot_start_ptes() collects the dirty
>>> and access bits across the batch, therefore batching across
>>> pte_dirty(): this is correct since the dirty bit on the PTE really is
>>> just an indication that the folio got written to, so even if the PTE is
>>> not actually dirty (but one of the PTEs in the batch is), the wp-fault
>>> optimization can be made.
>>>
>>> The crux now is how to batch around the PageAnonExclusive case; we must
>>> check the corresponding condition for every single page. Therefore, from
>>> the large folio batch, we process sub batches of ptes mapping pages with
>>> the same PageAnonExclusive condition, and process that sub batch, then
>>> determine and process the next sub batch, and so on. Note that this does
>>> not cause any extra overhead; if suppose the size of the folio batch
>>> is 512, then the sub batch processing in total will take 512 iterations,
>>> which is the same as what we would have done before.
>>>
>>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>>> ---
>>> mm/mprotect.c | 143 +++++++++++++++++++++++++++++++++++++++++---------
>>> 1 file changed, 117 insertions(+), 26 deletions(-)
>>>
>>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>>> index 627b0d67cc4a..28c7ce7728ff 100644
>>> --- a/mm/mprotect.c
>>> +++ b/mm/mprotect.c
>>> @@ -40,35 +40,47 @@
>>> #include "internal.h"
>>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>>> - pte_t pte)
>>> -{
>>> - struct page *page;
>>> +enum tristate {
>>> + TRI_FALSE = 0,
>>> + TRI_TRUE = 1,
>>> + TRI_MAYBE = -1,
>>> +};
>>> +/*
>>> + * Returns enum tristate indicating whether the pte can be changed to writable.
>>> + * If TRI_MAYBE is returned, then the folio is anonymous and the user must
>>> + * additionally check PageAnonExclusive() for every page in the desired range.
>>> + */
>>> +static int maybe_change_pte_writable(struct vm_area_struct *vma,
>>> + unsigned long addr, pte_t pte,
>>> + struct folio *folio)
>>> +{
>>> if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
>>> - return false;
>>> + return TRI_FALSE;
>>> /* Don't touch entries that are not even readable. */
>>> if (pte_protnone(pte))
>>> - return false;
>>> + return TRI_FALSE;
>>> /* Do we need write faults for softdirty tracking? */
>>> if (pte_needs_soft_dirty_wp(vma, pte))
>>> - return false;
>>> + return TRI_FALSE;
>>> /* Do we need write faults for uffd-wp tracking? */
>>> if (userfaultfd_pte_wp(vma, pte))
>>> - return false;
>>> + return TRI_FALSE;
>>> if (!(vma->vm_flags & VM_SHARED)) {
>>> /*
>>> * Writable MAP_PRIVATE mapping: We can only special-case on
>>> * exclusive anonymous pages, because we know that our
>>> * write-fault handler similarly would map them writable without
>>> - * any additional checks while holding the PT lock.
>>> + * any additional checks while holding the PT lock. So if the
>>> + * folio is not anonymous, we know we cannot change pte to
>>> + * writable. If it is anonymous then the caller must further
>>> + * check that the page is AnonExclusive().
>>> */
>>> - page = vm_normal_page(vma, addr, pte);
>>> - return page && PageAnon(page) && PageAnonExclusive(page);
>>> + return (!folio || folio_test_anon(folio)) ? TRI_MAYBE : TRI_FALSE;
>>> }
>>> VM_WARN_ON_ONCE(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte));
>>> @@ -80,15 +92,61 @@ bool can_change_pte_writable(struct vm_area_struct *vma,
>>> unsigned long addr,
>>> * FS was already notified and we can simply mark the PTE writable
>>> * just like the write-fault handler would do.
>>> */
>>> - return pte_dirty(pte);
>>> + return pte_dirty(pte) ? TRI_TRUE : TRI_FALSE;
>>> +}
>>> +
>>> +/*
>>> + * Returns the number of pages within the folio, starting from the page
>>> + * indicated by pgidx and up to pgidx + max_nr, that have the same value of
>>> + * PageAnonExclusive(). Must only be called for anonymous folios. Value of
>>> + * PageAnonExclusive() is returned in *exclusive.
>>> + */
>>> +static int anon_exclusive_batch(struct folio *folio, int pgidx, int max_nr,
>>> + bool *exclusive)
>>> +{
>>> + struct page *page;
>>> + int nr = 1;
>>> +
>>> + if (!folio) {
>>> + *exclusive = false;
>>> + return nr;
>>> + }
>>> +
>>> + page = folio_page(folio, pgidx++);
>>> + *exclusive = PageAnonExclusive(page);
>>> + while (nr < max_nr) {
>>> + page = folio_page(folio, pgidx++);
>>> + if ((*exclusive) != PageAnonExclusive(page))
>> nit: brackets not required around *exclusive.
>>
>>> + break;
>>> + nr++;
>>> + }
>>> +
>>> + return nr;
>>> +}
>>> +
>>> +bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>>> + pte_t pte)
>>> +{
>>> + struct page *page;
>>> + int ret;
>>> +
>>> + ret = maybe_change_pte_writable(vma, addr, pte, NULL);
>>> + if (ret == TRI_MAYBE) {
>>> + page = vm_normal_page(vma, addr, pte);
>>> + ret = page && PageAnon(page) && PageAnonExclusive(page);
>>> + }
>>> +
>>> + return ret;
>>> }
>>> static int mprotect_folio_pte_batch(struct folio *folio, unsigned long addr,
>>> - pte_t *ptep, pte_t pte, int max_nr_ptes)
>>> + pte_t *ptep, pte_t pte, int max_nr_ptes, fpb_t switch_off_flags)
>>> {
>>> - const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>> + fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>>> +
>>> + flags &= ~switch_off_flags;
>> This is mega confusing when reading the caller. Because the caller passes
>> FPB_IGNORE_SOFT_DIRTY and that actually means DON'T ignore soft dirty.
>>
>> Can't we just pass in the flags we want?
>>
>>> - if (!folio || !folio_test_large(folio) || (max_nr_ptes == 1))
>>> + if (!folio || !folio_test_large(folio))
>> What's the rational for dropping the max_nr_ptes == 1 condition? If you don't
>> need it, why did you add it in the earler patch?
>>
>>> return 1;
>>> return folio_pte_batch(folio, addr, ptep, pte, max_nr_ptes, flags,
>>> @@ -154,7 +212,8 @@ static int prot_numa_skip_ptes(struct folio **foliop,
>>> struct vm_area_struct *vma
>>> }
>>> skip_batch:
>>> - nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte, max_nr_ptes);
>>> + nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte,
>>> + max_nr_ptes, 0);
>>> out:
>>> *foliop = folio;
>>> return nr_ptes;
>>> @@ -191,7 +250,10 @@ static long change_pte_range(struct mmu_gather *tlb,
>>> if (pte_present(oldpte)) {
>>> int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
>>> struct folio *folio = NULL;
>>> - pte_t ptent;
>>> + int sub_nr_ptes, pgidx = 0;
>>> + pte_t ptent, newpte;
>>> + bool sub_set_write;
>>> + int set_write;
>>> /*
>>> * Avoid trapping faults against the zero or KSM
>>> @@ -206,6 +268,11 @@ static long change_pte_range(struct mmu_gather *tlb,
>>> continue;
>>> }
>>> + if (!folio)
>>> + folio = vm_normal_folio(vma, addr, oldpte);
>>> +
>>> + nr_ptes = mprotect_folio_pte_batch(folio, addr, pte, oldpte,
>>> + max_nr_ptes, FPB_IGNORE_SOFT_DIRTY);
>> From the other thread, my memory is jogged that this function ignores write
>> permission bit. So I think that's opening up a bug when applied here? If the
>> first pte is writable but the rest are not (COW), doesn't this now make them all
>> writable? I don't *think* that's a problem for the prot_numa use, but I could be
>> wrong.
>
> Can this be fixed by introducing FPB_HONOR_WRITE?
Yes I think so. Suddenly David's change looks very appealing because it's going
to say there are a set of bits that are ignored by default (young, dirty,
soft-dirty, write) and use FPB_HONOR_ flags to stop ignoring those bits. So we
can follow that pattern for write, I guess? And this avoids mixing FPB_IGNORE_
an FPB_HONOR_ flags.
>
>>
>>> oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes);
>> Even if I'm wrong about ignoring write bit being a bug, I don't think the docs
>> for this function permit write bit to be different across the batch?
>>
>>> ptent = pte_modify(oldpte, newprot);
>>> @@ -227,15 +294,39 @@ static long change_pte_range(struct mmu_gather *tlb,
>>> * example, if a PTE is already dirty and no other
>>> * COW or special handling is required.
>>> */
>>> - if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>>> - !pte_write(ptent) &&
>>> - can_change_pte_writable(vma, addr, ptent))
>>> - ptent = pte_mkwrite(ptent, vma);
>>> -
>>> - modify_prot_commit_ptes(vma, addr, pte, oldpte, ptent, nr_ptes);
>>> - if (pte_needs_flush(oldpte, ptent))
>>> - tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>>> - pages++;
>>> + set_write = (cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>>> + !pte_write(ptent);
>>> + if (set_write)
>>> + set_write = maybe_change_pte_writable(vma, addr, ptent, folio);
>> Why not just:
>> set_write = (cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>> !pte_write(ptent) &&
>> maybe_change_pte_writable(...);
>>
>> ?
>>
>>> +
>>> + while (nr_ptes) {
>>> + if (set_write == TRI_MAYBE) {
>>> + sub_nr_ptes = anon_exclusive_batch(folio,
>>> + pgidx, nr_ptes, &sub_set_write);
>>> + } else {
>>> + sub_nr_ptes = nr_ptes;
>>> + sub_set_write = (set_write == TRI_TRUE);
>>> + }
>>> +
>>> + if (sub_set_write)
>>> + newpte = pte_mkwrite(ptent, vma);
>>> + else
>>> + newpte = ptent;
>>> +
>>> + modify_prot_commit_ptes(vma, addr, pte, oldpte,
>>> + newpte, sub_nr_ptes);
>>> + if (pte_needs_flush(oldpte, newpte))
>> What did we conclude with pte_needs_flush()? I thought there was an arch where
>> it looked dodgy calling this for just the pte at the head of the batch?
>>
>> Thanks,
>> Ryan
>>
>>> + tlb_flush_pte_range(tlb, addr,
>>> + sub_nr_ptes * PAGE_SIZE);
>>> +
>>> + addr += sub_nr_ptes * PAGE_SIZE;
>>> + pte += sub_nr_ptes;
>>> + oldpte = pte_advance_pfn(oldpte, sub_nr_ptes);
>>> + ptent = pte_advance_pfn(ptent, sub_nr_ptes);
>>> + nr_ptes -= sub_nr_ptes;
>>> + pages += sub_nr_ptes;
>>> + pgidx += sub_nr_ptes;
>>> + }
>>> } else if (is_swap_pte(oldpte)) {
>>> swp_entry_t entry = pte_to_swp_entry(oldpte);
>>> pte_t newpte;